Original Antigenic Sin

Although numerous strains of influenza viruses may cause clinically indistinguishable forms of "grippe," the various agents may be reduced by serologic technics to two important types of virus, the influenza A's and their relatives, the B's; and two branches that are "country cousins," the C's and D's. Within the first two ancestral clans are perhaps a dozen or so families of the A strain and four or so major variants of B strains. Such genotypic distinctions are of more than academic interest since immunity established after infection by one type in no way protects against infection at a subsequent date . . .